Image pickup apparatus including infrared light reduction section,  method of controlling the same, and storage medium

ABSTRACT

An image pickup apparatus capable of accurately inserting/removing an infrared light reduction section. An infrared cut filter is inserted between a photographic optical system and an image pickup device to reduce infrared light contained in an optical image. If an object luminance acquired based on an optical image is lower than a first threshold, a camera control circuit calculates a second threshold for comparison with the object luminance and a third threshold value for comparison with an object luminance detected by a luminance sensor, based on the object luminances and the first threshold, and one of the second and third thresholds is made equal to or higher than the other. The infrared cut filter is selectively inserted or removed based on the object luminances, the second and third switching threshold values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup apparatus including aninfrared light reduction section that can be inserted into and removedfrom an optical path of a photographic optical system, a method ofcontrolling the same, and a storage medium, and more particularly to animage pickup apparatus that inserts or removes the infrared lightreduction section according to the luminance of an object.

2. Description of the Related Art

Conventionally, an image pickup apparatus, such as a digital camera,employs a method of selectively inserting or removing an infrared cutfilter which is an infrared light reduction section into or from anoptical path of a photographic optical system according to the luminanceof an object, and switching a photographing mode (hereinafter referredto as the automatic day/night function).

In the automatic day/night function, when the luminance of an object ishigher than a predetermined luminance, near infrared light (having awavelength of approximately 700 nm or more) is cut by inserting theinfrared cut filter into the optical path. On the other hand, when theluminance of an object becomes not higher than the predeterminedluminance, the infrared cut filter is removed from the optical path tothereby allow light in the near infrared region to pass and increase thesensitivity.

As described above, if light in the near infrared region is allowed topass, the color balance of an image is lost, and hence when the infraredcut filter is removed, it is necessary to switch the photographing modefrom a color image mode (day mode) to a monochrome image mode (nightmode).

Note that the luminance of an object is detected, for example, based onan image signal which is output from an image pickup device, such as aCMOS image sensor, or using a luminance sensor provided separately fromthe photographic optical system. In a case where the luminance of anobject is detected using the luminance sensor, if a photographing rangethrough the photographic optical system and a range of luminancedetection by the luminance sensor are different, the luminance of anobject is not accurately detected, which makes it impossible to properlyswitch the photographing mode.

To cope with this problem, for example, there has been proposed a methodof correcting the range of luminance detection according to a change inthe angle of view of a zoom lens provided in the image pickup opticalsystem to thereby accurately detect the luminance of an object andstabilize switching of the photographing mode (see Japanese PatentLaid-Open Publication No. 2010-279061 and Japanese Patent Laid-OpenPublication No. 2007-49442).

However, in the methods described in Japanese Patent Laid-OpenPublication No. 2010-279061 and Japanese Patent Laid-Open PublicationNo. 2007-49442, if the infrared light reflectivity is not uniform in thephotographing range through the photographic optical system, an erroroccurs in correction of the range of luminance detection. Therefore, forexample, in a photographing environment in which an object isilluminated with infrared illumination, the infrared light reflectivityis nonuniform in the photographing range through the photographicoptical system due to the influence of an object distance or the like,and as a result, it is impossible to stably switch the photographingmode.

Further, there has been proposed an image pickup apparatus that includesan infrared illumination section and illuminates an object with infraredlight using the infrared illumination section when in the night mode.This image pickup apparatus positively photographs an object even underlow luminance by illuminating the object with infrared light (seeJapanese Patent Laid-Open Publication No. 2004-229034).

Incidentally, when the photographing mode is switched based on theobject luminance obtained according to the image signal output from theimage pickup device, the signal level of the image signal largely variesbetween a day mode and a night mode. This causes a so-called huntingphenomenon in which the photographing mode is repeatedly switched withina short time.

To prevent the hunting phenomenon, it is envisaged to switch thephotographing mode to the day mode in a case where the object luminancebecomes higher than an object luminance obtained after switching thephotographing mode to the night mode. However, in the image pickupapparatus including the infrared illumination section, if the intensityof light emission by the infrared illumination section changes, it isdifficult to switch the photographing mode at a desired timing.

On the other hand, in the image pickup apparatus described in JapanesePatent Laid-Open Publication No. 2004-229034, although the luminance ofan object is detected using the luminance sensor provided separatelyfrom the photographic optical system, the image pickup apparatus isrequired to be additionally provided with the luminance sensor, whichinevitably increases the costs of the image pickup apparatus itself.

SUMMARY OF THE INVENTION

The present invention provides an image pickup apparatus that is capableof accurately inserting and removing an infrared light reductionsection, a method of controlling the same, and a storage medium.

Further, the present invention provides an image pickup apparatus thatis capable of switching the photographing mode at a desired timingwithout causing a hunting phenomenon even when the photographing mode isswitched in a case where photographing is performed by illumination withinfrared light, a method of controlling the same, and a storage medium.

In a first aspect of the present invention, there is provided an imagepickup apparatus including an image pickup device that has an opticalimage formed thereon through a photographic optical system, and outputsan image signal corresponding to the optical image, comprising aninfrared light reduction unit configured to be inserted between thephotographic optical system and the image pickup device so as to reducean amount of infrared light contained in the optical image reaching theimage pickup device, a first detection unit configured to detect aluminance of an object according to the image signal to thereby obtain afirst detection result, a second detection unit disposed at a locationdifferent from a location of the image pickup device and configured todetect a luminance of the object to thereby obtain a second detectionresult, a processing unit configured to, in a case where the firstdetection result is lower than a first switching threshold value set inadvance, generate a second switching threshold value used for comparisonwith the first detection result and a third switching threshold valueused for comparison with the second detection result, based on the firstdetection result, the second detection result, and the first switchingthreshold value, and make one of the second switching threshold valueand the third switching threshold value equal to or higher than theother of the second switching threshold value and the third switchingthreshold value, and an insertion/removal control unit configured toperform insertion/removal control for selectively inserting or removingthe infrared light reduction unit into or from an optical path betweenthe photographic optical system and the image pickup device, based onthe first detection result, the second detection result, the secondswitching threshold value, and the third switching threshold value.

In a second aspect of the present invention, there is provided an imagepickup apparatus including an image pickup device that has an opticalimage formed thereon through a photographic optical system, and outputsan image signal corresponding to the optical image, comprising aninfrared light reduction unit configured to be inserted between thephotographic optical system and the image pickup device so as to reducean amount of infrared light contained in the optical image reaching theimage pickup device, a first detection unit configured to detect aluminance of an object according to the image signal to thereby obtain afirst detection result, a second detection unit disposed at a locationdifferent from a location of the image pickup device and configured todetect a luminance of the object to thereby obtain a second detectionresult, a selection unit configured to compare the first detectionresult and the second detection result, and select one of the firstdetection result and the second detection result as a selected detectionresult based on a comparison result, and an insertion/removal controlunit configured to perform insertion/removal control for selectivelyinserting or removing the infrared light reduction unit into or from anoptical path between the photographic optical system and the imagepickup device according to the selected detection result.

In a third aspect of the present invention, there is provided an imagepickup apparatus including an image pickup device that has an opticalimage formed thereon through a photographic optical system, and outputsan image signal corresponding to the optical image, comprising aninfrared light reduction unit configured to be inserted into an opticalpath between the photographic optical system and the image pickup deviceso as to reduce an amount of infrared light contained in the opticalimage reaching the image pickup device, a detection unit configured todetect a luminance of an object according to the image signal to therebyobtain a luminance detection result, an illuminating unit configured toilluminate the object with infrared light, a correction unit configuredto correct a threshold value for use in determining insertion or removalof the infrared light reduction unit into or from the optical path,based on a light emission intensity of infrared light emitted by theilluminating unit, and a control unit configured to compare thedetection result and a corrected threshold value, and determineinsertion of the infrared light reduction unit into the optical pathaccording to the comparison result.

In a fourth aspect of the present invention, there is provided a methodof controlling an image pickup apparatus including an image pickupdevice that has an optical image formed thereon through a photographicoptical system, and outputs an image signal corresponding to the opticalimage, and an infrared light reduction unit configured to be insertedbetween the photographic optical system and the image pickup device soas to reduce an amount of infrared light contained in the optical imagereaching the image pickup device, the method comprising detecting aluminance of an object according to the image signal to thereby obtain afirst detection result, detecting a luminance of the object using aluminance sensor disposed at a location different from a location of theimage pickup device to thereby obtain a second detection result,generating, in a case where the first detection result is lower than afirst switching threshold value set in advance, a second switchingthreshold value used for comparison with the first detection result anda third switching threshold value used for comparison with the seconddetection result, based on the first detection result, the seconddetection result, and the first switching threshold value, and makingone of the second switching threshold value and the third switchingthreshold value equal to or higher than the other of the secondswitching threshold value and the third switching threshold value, andperforming insertion/removal control for selectively inserting orremoving the infrared light reduction unit into or from an optical pathbetween the photographic optical system and the image pickup device,based on the first detection result, the second detection result, thesecond switching threshold value, and the third switching thresholdvalue.

In a fifth aspect of the present invention, there is provided a methodof controlling an image pickup apparatus including an image pickupdevice that has an optical image formed thereon through a photographicoptical system, and outputs an image signal corresponding to the opticalimage, and an infrared light reduction unit configured to be insertedbetween the photographic optical system and the image pickup device soas to reduce an amount of infrared light contained in the optical imagereaching the image pickup device, the method comprising detecting aluminance of an object according to the image signal to thereby obtain afirst detection result, detecting a luminance of the object using aluminance sensor disposed at a location different from a location of theimage pickup device to thereby obtain a second detection result,comparing the first detection result and the second detection result,and selecting one of the first detection result and the second detectionresult as a selected detection result based on a comparison result, andperforming insertion/removal control for selectively inserting orremoving the infrared light reduction unit into or from an optical pathbetween the photographic optical system and the image pickup deviceaccording to the selected detection result.

In a sixth aspect of the present invention, there is provided a methodof controlling an image pickup apparatus including an image pickupdevice that has an optical image formed thereon through a photographicoptical system, and outputs an image signal corresponding to the opticalimage, an infrared light reduction unit configured to be inserted intoan optical path between the photographic optical system and the imagepickup device so as to reduce an amount of infrared light contained inthe optical image reaching the image pickup device, and an illuminatingunit configured to illuminate the object with infrared light, the methodcomprising detecting a luminance of an object according to the imagesignal to thereby obtain a luminance detection result, correcting athreshold value for use in determining insertion or removal of theinfrared light reduction unit into or from the optical path, based on alight emission intensity of infrared light emitted by the illuminatingunit, and comparing the detection result and a corrected thresholdvalue, and determining insertion of the infrared light reduction unitinto the optical path according to the comparison result.

In a seventh aspect of the present invention, there is provided anon-transitory computer-readable storage medium storing acomputer-executable control program for causing a computer to execute amethod of controlling an image pickup apparatus including an imagepickup device that has an optical image formed thereon through aphotographic optical system, and outputs an image signal correspondingto the optical image, and an infrared light reduction unit configured tobe inserted between the photographic optical system and the image pickupdevice so as to reduce an amount of infrared light contained in theoptical image reaching the image pickup device, wherein the methodcomprises detecting a luminance of an object according to the imagesignal to thereby obtain a first detection result, detecting a luminanceof the object using a luminance sensor disposed at a location differentfrom a location of the image pickup device to thereby obtain a seconddetection result, generating, in a case where the first detection resultis lower than a first switching threshold value set in advance, a secondswitching threshold value used for comparison with the first detectionresult and a third switching threshold value used for comparison withthe second detection result, based on the first detection result, thesecond detection result, and the first switching threshold value, andmaking one of the second switching threshold value and the thirdswitching threshold value equal to or higher than the other of thesecond switching threshold value and the third switching thresholdvalue, and performing insertion/removal control for selectivelyinserting or removing the infrared light reduction unit into or from anoptical path between the photographic optical system and the imagepickup device, based on the first detection result, the second detectionresult, the second switching threshold value, and the third switchingthreshold value.

In an eighth aspect of the present invention, there is provided anon-transitory computer-readable storage medium storing acomputer-executable control program for causing a computer to execute amethod of controlling an image pickup apparatus including an imagepickup device that has an optical image formed thereon through aphotographic optical system, and outputs an image signal correspondingto the optical image, and an infrared light reduction unit configured tobe inserted between the photographic optical system and the image pickupdevice so as to reduce an amount of infrared light contained in theoptical image reaching the image pickup device, wherein the methodcomprises detecting a luminance of an object according to the imagesignal to thereby obtain a first detection result, detecting a luminanceof the object using a luminance sensor disposed at a location differentfrom a location of the image pickup device to thereby obtain a seconddetection result, comparing the first detection result and the seconddetection result, and selecting one of the first detection result andthe second detection result as a selected detection result based on acomparison result, and performing insertion/removal control forselectively inserting or removing the infrared light reduction unit intoor from an optical path between the photographic optical system and theimage pickup device according to the selected detection result.

In a ninth aspect of the present invention, there is provided anon-transitory computer-readable storage medium storing acomputer-executable control program for causing a computer to execute amethod of controlling an image pickup apparatus including an imagepickup device that has an optical image formed thereon through aphotographic optical system, and outputs an image signal correspondingto the optical image, an infrared light reduction unit configured to beinserted into an optical path between the photographic optical systemand the image pickup device so as to reduce an amount of infrared lightcontained in the optical image reaching the image pickup device, and anilluminating unit configured to illuminate the object with infraredlight, wherein the method comprises detecting a luminance of an objectaccording to the image signal to thereby obtain a luminance detectionresult, correcting a threshold value for use in determining insertion orremoval of the infrared light reduction unit into or from the opticalpath, based on a light emission intensity of infrared light emitted bythe illuminating unit, and comparing the detection result and acorrected threshold value, and determining insertion of the infraredlight reduction unit into the optical path according to the comparisonresult.

According to the present invention, it is possible to accurately andstably insert and remove the infrared light reduction section. Further,in the case where photographing is performed by illumination withinfrared light, it is possible to switch the photographing mode at adesired timing without causing a hunting phenomenon even when thephotographing mode is switched.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a digital camera as an image pickupapparatus according to a first embodiment of the present invention.

FIG. 2 is a block diagram of a control circuit appearing in FIG. 1.

FIG. 3 is a diagram useful in explaining a relationship between aphotographing range and a range of luminance detection by a luminancesensor in the digital camera shown in FIG. 1.

FIG. 4 is a diagram useful in explaining another example of therelationship between the photographing range and the range of luminancedetection by the luminance sensor in the digital camera shown in FIG. 1.

FIG. 5 is a flowchart of a mode determination process for a modeswitching operation performed by the digital camera shown in FIG. 1.

FIG. 6 is a schematic diagram of a digital camera as an image pickupapparatus according to a second embodiment.

FIG. 7 is a block diagram of a control circuit appearing in FIG. 6.

FIG. 8 is a flowchart of a mode determination process for a modeswitching operation performed by the digital camera shown in FIG. 6.

FIG. 9 is a flowchart of a first variation of the mode determinationprocess for the mode switching operation performed by the digital camerashown in FIG. 6.

FIG. 10 is a flowchart of a second variation of the mode determinationprocess for the mode switching operation performed by the digital camerashown in FIG. 6.

FIG. 11 is a schematic diagram of a digital camera as an image pickupapparatus according to a third embodiment.

FIG. 12 is a block diagram of a control circuit appearing in FIG. 11.

FIG. 13 is a flowchart of a mode determination process for a modeswitching operation performed by the digital camera shown in FIG. 11.

DESCRIPTION OF THE EMBODIMENTS

The present invention will now be described in detail below withreference to the accompanying drawings showing embodiments thereof.

FIG. 1 is a schematic diagram of an image pickup apparatus according toa first embodiment of the present invention.

The image pickup apparatus shown in FIG. 1 is e.g. a digital camera(hereinafter simply referred to as the camera), and includes a pluralityof photographing lenses 1 which form a photographic optical system. Afilter switching mechanism 3 is arranged downstream of the photographicoptical system, and is provided with a filter part including an infraredcut filter 4 (infrared light reduction unit) and a transparent substrate5. The infrared cut filter 4 may be a filter that completely blocksinfrared light, or a filter that does not completely block infraredlight but provides a sufficient light reduction effect with a lightshielding ratio of not lower than a predetermined value (e.g. not lowerthan 90%), insofar as it reduces an amount of infrared light reaching animage pickup device 7.

The filter part is movable in a direction orthogonal to (intersectingwith) an optical axis (i.e. an optical path) 2 of the photographicoptical system, and the infrared cut filter 4 or the transparentsubstrate 5 is selectively positioned on the optical axis 2 inaccordance with the movement of the filter part. More specifically, aninfrared cut filter insertion/removal motor 6 is driven under thecontrol of a control circuit 10, whereby the infrared cut filter 4 orthe transparent substrate 5 can be selectively inserted onto or removedfrom the optical axis 2 (in other words, the infrared cut filter 4 canbe inserted into or removed from the optical path).

The image pickup device 7, such as a CMOS image sensor, is provideddownstream of the filter switching mechanism 3, and an optical image(object image) is formed on the image pickup device 7 through thephotographic optical system. Then, the image pickup device 7 outputs animage signal corresponding to the optical image to the control circuit10.

A luminance sensor 8 is connected to the control circuit 10. Theluminance sensor 8 receives light from a direction of an optical axis 9thereof, and detects the luminance within a photographing range throughthe photographic optical system. The luminance sensor 8 hassubstantially the same spectral sensitivity characteristics as those ofthe image pickup device 7 and the infrared cut filter 4 exhibited whenthe infrared cut filter 4 is positioned on the optical axis 2.

The control circuit 10 controls the overall operation of the camera. Thecontrol circuit 10 controls input of power supply, and outputs a videosignal corresponding to an image signal.

At least one of the photographing lenses 1 is a zoom lens, and thecontrol circuit 10 controls the driving of a zoom motor 12 to therebymove the photographing lenses 1 along the optical axis 2 and therebychange the photographing lenses 1 in a range between a wide angle endand a telephoto end.

FIG. 2 is a block diagram of the control circuit 10 appearing in FIG. 1.Note that in FIG. 2, the filter switching mechanism 3 appearing in FIG.1 is omitted and only one photographing lens 1 is illustrated.

The control circuit 10 includes a camera control circuit 21, an imagepickup device control circuit 22, a luminance sensor control circuit 23,a video signal processing circuit 24, an infrared cut filter controlcircuit 25, a photometric circuit 26, a video signal output circuit 27,and a zoom motor control circuit 29.

In photographing, an optical image incident through the photographinglenses 1 passes through the IR cut filter 4 or the transparent substrate5 and is formed on the image pickup device 7. The image pickup devicecontrol circuit 22 controls reading from the image pickup device 7 tothereby cause the image pickup device 7 to output an image signalcorresponding to the optical image. Then, the image pickup devicecontrol circuit 22 sends the image signal to the photometric circuit 26and the video signal processing circuit 24.

The video signal processing circuit 24 performs predetermined signalprocessing on the image signal, and thereby generates a color ormonochrome video signal. Then, this video signal is output to theoutside by the video signal output circuit 27.

The photometric circuit 26 measures a luminance of an object accordingto the image signal, and sends a first detection result indicative ofthe luminance measured thereby to the camera control circuit 21. Theluminance sensor 8 is controlled by the luminance sensor control circuit23, and the luminance sensor control circuit 23 sends a second detectionresult indicative of a luminance detected by the luminance sensor 8 tothe camera control circuit 21.

The camera control circuit 21 controls the infrared cut filter controlcircuit 25 according to the first and second detection results. Withthis control, the infrared cut filter control circuit 25 controlsdriving of the infrared cut filter insertion/removal motor 6.

The camera control circuit 21 controls driving of the zoom motor 12 bythe zoom motor control circuit 29 to thereby move the photographinglenses 1 along the optical axis 2 and thereby change the photographinglenses 1 in the range between the wide angle end and the telephoto end.

FIG. 3 is a diagram useful in explaining a relationship between thephotographing range and the range of luminance detection by theluminance sensor 8, in the camera shown in FIG. 1.

FIG. 3 shows an example of a case where the photographing lenses 1 isset to the wide angle end, and in this case, the photographing range,denoted by reference numeral 31, is larger than the range of luminancedetection, denoted by reference numeral 32.

FIG. 4 is a diagram useful in explaining another example of therelationship between the photographing range and the range of luminancedetection by the luminance sensor 8, in the camera shown in FIG. 1.

FIG. 4 shows an example of a case where the photographing lenses 1 isset to the telephoto end, and in this case, the photographing range,denoted by reference numeral 41, is smaller than the range of luminancedetection, denoted by reference numeral 42.

Here, a description will be given of an operation performed by thecamera shown in FIG. 1, for switching the photographing mode between aday mode (color photographing mode) and a night mode (monochromephotographing mode).

FIG. 5 is a flowchart of a mode determination process for the modeswitching operation performed by the camera shown in FIG. 1. The modedetermination process in FIG. 5 is performed under the control of thecamera control circuit 21, and is automatically and repeatedly executedwhen the camera is in a power-on state, for example.

When the mode determination process is started, the camera controlcircuit 21 obtains the first detection result (object luminance Y1) fromthe photometric circuit 26 (step S501). Then, the camera control circuit21 obtains the second detection result (object luminance Y2) from theluminance sensor control circuit 23 (step S502).

Next, the camera control circuit 21 determines whether or not thecurrent photographing mode is the day mode (step S503). If the currentphotographing mode is the day mode (YES to the step S503), the cameracontrol circuit 21 compares the object luminance Y1 with a firstswitching threshold value Yth1 set in advance, and determines whether ornot the object luminance Y1<the first switching threshold value Yth1holds (step S504).

Note that the first switching threshold value Yth1 is a threshold valuewith reference to which the photographing mode is switched from the daymode to the night mode.

If the object luminance Y1<the first switching threshold value Yth1holds (i.e. if the object luminance Y1 is lower than the first switchingthreshold value) (YES to the step S504), the camera control circuit 21calculates a difference Ydiff between the object luminances Y1 and Y2(step S505). Then, the camera control circuit 21 switches thephotographing mode from the day mode to the night mode (step S506). Atthis time, in accordance with the switching from the day mode to thenight mode, the camera control circuit 21 controls the infrared cutfilter control circuit 25 to remove the infrared cut filter 4 from theoptical axis 2.

Then, the camera control circuit 21 compares the difference Ydiff with adifference threshold value Yth4 set in advance and compares the objectluminance Y1 with the object luminance Y2, to thereby determine based onthe comparison results whether or not the difference Ydiff<thedifference threshold value Yth4 or the object luminance Y1>the objectluminance Y2 holds (step S507).

If the difference Ydiff<the difference threshold value Yth4 or theobject luminance Y1>the object luminance Y2 holds (YES to the stepS507), the camera control circuit 21 calculates a second switchingthreshold value Yth2 based on the first switching threshold value Yth1(step S508).

The second switching threshold value Yth2 is a threshold value withreference to which the photographing mode is switched from the nightmode to the day mode, and for example, the second switching thresholdvalue Yth2 is calculated by an equation Yth2=Yth1+α.

The above-mentioned variable α represents hysteresis for preventingoccurrence of hunting in switching the photographing mode, and when thevariable α is set to a larger value, it is possible to more proactivelyprevent occurrence of hunting. On the other hand, when the variable α isset to a smaller value, it is possible to more quickly switch thephotographing mode from the night mode to the day mode.

Next, the camera control circuit 21 obtains the object luminance Y2detected by the luminance sensor 8 from the luminance sensor controlcircuit 23 again (step S509). The step S509 is performed because theobject luminance Y2 in the step S502 is sometimes changed by theinfluence of infrared light on the output from the luminance sensor 8.

Then, the camera control circuit 21 calculates a third switchingthreshold value Yth3 based on the object luminance Y2 which is obtainedin the step S509 and with reference to which the photographing mode isswitched from the night mode to the day mode (step S510). Here, thecamera control circuit 21 calculates the third switching threshold valueYth3 e.g. by an equation Yth3=Y2+α.

Then, the camera control circuit 21 compares the second switchingthreshold value Yth2 the third switching threshold value Yth3 with eachother, and determines whether or not the third switching threshold valueYth3<the second switching threshold value Yth2 holds (step S511).

If the third switching threshold value Yth3<the second switchingthreshold value Yth2 holds (YES to the step S511), the camera controlcircuit 21 makes the third switching threshold value Yth3 equal to thesecond switching threshold value Yth2 (step S512). Then, the cameracontrol circuit 21 terminates the mode determination process.

On the other hand, if the third switching threshold value Yth3≧thesecond switching threshold value Yth2 holds (NO to the step S511), thecamera control circuit 21 terminates the mode determination process.With these processing operations, the third switching threshold valueYth3 is made always equal to or higher than the second switchingthreshold value Yth2.

If the difference Ydiff≧the difference threshold value Yth4 holds andalso the object luminance Y1≦the object luminance Y2 (the firstdetection result is not higher than the second detection result) holds(NO to the step S507), the camera control circuit 21 calculates thethird switching threshold value Yth3 based on the first switchingthreshold value Yth1 (step S513).

The third switching threshold value Yth3 is a threshold value withreference to which the photographing mode is switched from the nightmode to the day mode as mentioned above, and for example, the thirdswitching threshold value Yth3 is calculated e.g. by an equationYth3=Yth1+α.

Next, the camera control circuit 21 obtains the object luminance Y1detected based the image signal from the photometric circuit 26 again(step S514). The step S514 is executed because the object luminance Y1obtained in the step S501 changes due to switching of the photographingmode to the night mode in which light including near infrared light isreceived.

Then, the camera control circuit 21 calculates the second switchingthreshold value Yth2 based on the object luminance Y1 obtained in thestep S514 (step S515). Here, the camera control circuit 21 calculatesthe second switching threshold value Yth2 e.g. by an equation Yth2=Y1+α.

Then, the camera control circuit 21 compares the second switchingthreshold value Yth2 and the third switching threshold value Yth3 witheach other, and determines whether or not the second switching thresholdvalue Yth2<the third switching threshold value Yth3 holds (step S516).

If the second switching threshold value Yth2<the third switchingthreshold value Yth3 holds (YES to the step S516), the camera controlcircuit 21 makes the second switching threshold value Yth2 equal to thethird switching threshold value Yth3 (step S517). Then, the cameracontrol circuit 21 terminates the mode determination process.

On the other hand, if the second switching threshold value Yth2≧thethird switching threshold value Yth3 holds (NO to the step S516), thecamera control circuit 21 terminates the mode determination process.With these processing operations, the second switching threshold valueYth2 is always made equal to or higher than the third switchingthreshold value Yth3.

Thus, in the steps S508 to S517, processing for making one of the secondswitching threshold value and the third switching threshold value equalto or higher than the other is performed.

If the object luminance Y1 the first switching threshold value Yth1holds (NO to the step S504), the camera control circuit 21 terminatesthe mode determination process.

If the current photographing mode is not the day mode (NO to the stepS503), i.e. if the photographing mode is the night mode, the cameracontrol circuit 21 compares the object luminance Y1 with the secondswitching threshold value Yth2, and compares the object luminance Y2with the third switching threshold value Yth3. Then, the camera controlcircuit 21 determines based on these comparison results whether or notthe object luminance Y1>the second switching threshold value Yth2 holdsand also the object luminance Y2>the third switching threshold valueYth3 holds (step S518).

If the object luminance Y1>the second switching threshold value Yth2holds and also the object luminance Y2>the third switching thresholdvalue Yth3 holds (YES to the step S518), the camera control circuit 21switches the photographing mode from the night mode to the day mode(step S519). At this time, in accordance with the switching from thenight mode to the day mode, the camera control circuit 21 controls theinfrared cut filter control circuit 25 to insert the infrared cut filter4 onto the optical axis 2. Then, the camera control circuit 21terminates the mode determination process.

On the other hand, if the object luminance Y1≦the second switchingthreshold value Yth2 or the object luminance Y2≦the third switchingthreshold value Yth3 holds (NO to the step S518), the camera controlcircuit 21 terminates the mode determination process.

As described above, in the first embodiment, the second switchingthreshold value Yth2 and the third switching threshold value Yth3 areadjusted based on the difference Ydiff between the object luminance Y1and the object luminance Y2.

In this adjustment, if the difference Ydiff is smaller than thepredetermined difference threshold value Yth4 (smaller than thedifference threshold value), it is judged that reliability of the objectluminance Y2 is high, and the camera control circuit 21 makes the thirdswitching threshold value Yth3 equal to or higher than the secondswitching threshold value Yth2, and switches the photographing mode fromthe night mode to the day mode while attaching importance to the objectluminance Y2. That is, the camera control circuit 21 inserts theinfrared cut filter 4 onto the optical axis 2 while attaching importanceto the object luminance Y2.

On the other hand, if the difference Ydiff is not smaller than thepredetermined difference threshold value Yth4 (not smaller than thedifference threshold value), it is judged that reliability of the objectluminance Y2 is low, and the camera control circuit 21 makes the secondswitching threshold value Yth2 equal to or higher than the thirdswitching threshold value Yth3, and switches the photographing mode fromthe night mode to the day mode while attaching importance to the objectluminance Y1. That is, the camera control circuit 21 inserts theinfrared cut filter 4 onto the optical axis 2 while attaching importanceto the object luminance Y1.

As a result, in the first embodiment, even when the photographing rangethrough the photographic optical system and the range of luminancedetection by the luminance sensor are different, and the infrared lightreflectivity in the photographic range is not uniform, it is possible tostabilize switching of the photographing mode. Further, it is possibleto accurately and stably insert and remove the infrared cut filter 4.

Although in the above-described first embodiment, the difference iscalculated when comparing the object luminance Y1 and the objectluminance Y2 with each other, in place of the difference, a ratiobetween the object luminance Y1 and the object luminance Y2, forexample, may be used.

More specifically, in a case where a first determination result isobtained which indicates either that the ratio between the objectluminance Y1 and the object luminance Y2 is lower than a ratio thresholdvalue set in advance, or that the object luminance Y1 is higher than theobject luminance Y2, the second switching threshold value is generatedbased on the first switching threshold value, and the third switchingthreshold value is generated based on the object luminance Y2. Then,first processing is performed in which the third switching thresholdvalue is made equal to or higher than the second switching thresholdvalue.

Further, in a case where a second determination result is obtained whichindicates that the ratio between the object luminance Y1 and the objectluminance Y2 is not lower than the ratio threshold value, and also thatthe object luminance Y1 is not higher than the object luminance Y2, thethird switching threshold value is generated based on the firstswitching threshold value, and the second switching threshold value isgenerated based on the object luminance Y1. Then, second processing isperformed in which the second switching threshold value is made equal toor higher than the third switching threshold value.

Next, a description will be given of a camera as an image pickupapparatus according to a second embodiment of the present invention.Note that also in the camera according to the second embodiment, thephotographing range and the range of luminance detection have the samerelationship as shown in FIGS. 3 and 4.

FIG. 6 is a schematic diagram of the camera as the image pickupapparatus according to the second embodiment. The same components of thecamera shown in FIG. 6 as those of the camera shown in FIG. 1 aredenoted by the same reference numerals, and description thereof isomitted.

The control circuit 10 has not only the luminance sensor 8 but also aninfrared illumination section 11 connected thereto, and the infraredillumination section 11 illuminates an object with infrared light usinge.g. an LED as a light source under the control of the control circuit10.

FIG. 7 is a block diagram of the control circuit 10 appearing in FIG. 6.The same components of the control circuit shown in FIG. 7 as those ofthe control circuit shown in FIG. 2 are denoted by the same referencenumerals, and description thereof is omitted.

The control circuit 10 includes an infrared illumination control circuit28 in addition to the component elements described with reference toFIG. 2. The camera control circuit 21 controls the infrared illuminationcontrol circuit 28 according to the first and second detection results.With this control, the infrared illumination control circuit 28 controlsdriving of the infrared illumination section 11.

FIG. 8 is a flowchart of a mode determination process for the modeswitching operation performed by the camera shown in FIG. 6. The samesteps of the mode determination process in FIG. 8 as those of the modedetermination process in FIG. 5 are denoted by the same step numbers,and description thereof is omitted.

After the photographing mode has been switched from the day mode to thenight mode in the step S506, the camera control circuit 21 determineswhether or not infrared light is being emitted by the infraredillumination section 11 (step S801). If infrared light is being emittedby the infrared illumination section 11 (YES to the step S801), thecamera control circuit 21 proceeds to the step S507.

If infrared light is not being emitted by the infrared illuminationsection 11 (NO to the step S801), the camera control circuit 21 obtainsthe object luminance Y1 detected according to the image signal from thephotometric circuit 26 again (step S802). The step S802 is performedbecause the object luminance Y1 obtained in the step S501 changes due toswitching of the photographing mode to the night mode in which lightincluding near infrared light is received.

Then, the camera control circuit 21 calculates the second switchingthreshold value Yth2 based on the object luminance Y1 obtained in thestep S802 (step S803). Here, the camera control circuit 21 calculatesthe second switching threshold value Yth2 e.g. by the equationYth2=Y1+α. Then, the camera control circuit 21 terminates the modedetermination process.

If the current photographing mode is not the day mode (NO to the stepS503), i.e. if the current photographing mode is the night mode, thecamera control circuit 21 determines whether or not infrared light isbeing emitted by the infrared illumination section 11 (step S804). Ifinfrared light is being emitted by the infrared illumination section 11(YES to the step S804), the camera control circuit 21 proceeds to theabove-described step S518.

On the other hand, if infrared light is not being emitted by theinfrared illumination section (NO to the step S804), the camera controlcircuit 21 determines whether or not the object luminance Y1>the secondswitching threshold value Yth2 holds (step S805). If the objectluminance Y1>the second switching threshold value Yth2 holds (YES to thestep S805), the camera control circuit 21 proceeds to the step S519.

If the object luminance Y1≦the second switching threshold value Yth2holds (NO to the step S805), the camera control circuit 21 terminatesthe mode determination process.

As described above, in the second embodiment, in a case where infraredillumination is not being performed, it is judged that an error in theobject luminance Y1 is small, and the switching control for switchingthe photographing mode from the night mode to the day mode is performedbased on the object luminance Y1 and the second switching thresholdvalue Yth2. That is, the camera control circuit 21 inserts the infraredcut filter 4 onto the optical axis 2 based on the object luminance Y1and the second switching threshold value Yth2.

With this, in a case where infrared illumination is not being performed,it is possible to stably switch the photographing mode regardless of adifference between the photographic range through the photographicoptical system and the range of luminance detection by the luminancesensor. Further, it is possible to accurately and stably insert andremove the infrared cut filter 4.

Although also in the above-described second embodiment, the differenceis calculated when comparing the object luminance Y1 and the objectluminance Y2 with each other, similarly to the first embodiment, inplace of the difference, a ratio between the object luminance Y1 and theobject luminance Y2, for example, may be used for switching thephotographing mode.

As is clear from the above, in the first and second embodiments, thephotometric circuit 26 functions as a first detection unit, and theluminance sensor 8 and the luminance sensor control circuit 23 functionas a second detection unit.

Further, the camera control circuit 21 functions as a processing unitand a mode switching control unit, and the camera control circuit 21 andthe infrared cut filter insertion/removal motor 6 function as aninsertion/removal control unit.

FIG. 9 is a flowchart of a first variation of the mode determinationprocess for the mode switching operation performed by the camera shownin FIG. 6. The same steps of the mode determination process in FIG. 9 asthose of the mode determination process in FIG. 8 are denoted by thesame step numbers, and description thereof is omitted.

In the mode determination process in FIG. 9, if it is determined in thestep S503 that the photographing mode is not the day mode (NO to thestep S503), i.e. if the current photographing mode is the night mode, ina step S504′ corresponding to the step S504 of the mode determinationprocess in FIG. 8, the camera control process 21 compares the objectluminance Y1 with a luminance threshold value Yn set in advance, anddetermines whether or not the object luminance Y1<the luminancethreshold value Yn holds.

If the object luminance Y1<the luminance threshold value Yn holds (ifthe object luminance Y1 is smaller than the luminance threshold value)(YES to the step S504′), the camera control circuit 21 calculates thedifference Ydiff between the object luminance Y1 and the objectluminance Y2 in the step S505, switches the photographing mode from theday mode to the night mode in the step S506, and then proceeds to thestep S801.

As a result of the determination of whether or not infrared light isbeing emitted by the infrared illumination section 11 in the step S801,if it is determined that infrared light is being emitted by the infraredillumination section 11 (YES to the step S801), the camera controlcircuit 21 compares the difference Ydiff with a difference thresholdvalue Yth set in advance, and determines based on the comparison resultwhether or not the difference Ydiff<the difference threshold value Ythholds (step S901).

If the difference Ydiff<the difference threshold value Yth holds (i.e.if the difference is smaller than the difference threshold value) (YESto the step S901), the camera control circuit 21 stores thedetermination result in an internal memory thereof as a firstdetermination result (step S902). Then, the camera control circuit 21obtains the object luminance Y2 detected by the luminance sensor 8 fromthe luminance sensor control circuit 23 again (step S903).

The step S903 is performed because infrared light is being emitted bythe infrared illumination section 11, and this is because the objectluminance Y2 obtained in the step S502 is changed by the influence ofinfrared illumination on the output from the luminance sensor 8.

Then, the camera control circuit 21 calculates, based on the objectluminance Y2 obtained in the step S903, a first switching thresholdvalue Yd1 with reference to which the photographing mode is switchedfrom the night mode to the day mode (step S904).

In this step, the camera control circuit 21 calculates the firstswitching threshold value Yd1 e.g. by an equation Yd1=Y2+α. After thestep S904, the camera control circuit 21 terminates the modedetermination process.

The above-mentioned variable α represents hysteresis for preventingoccurrence of hunting in switching the photographing mode, and when thevariable α is set to a larger value, it is possible to more proactivelyprevent occurrence of hunting. On the other hand, when the variable α isset to a smaller value, it is possible to more quickly switch thephotographing mode from the night mode to the day mode.

Note that in a case where the first switching threshold value Yd1 is nothigher than the luminance threshold value Yn, after the photographingmode is switched from the night mode to the day mode, there is a fearthat the answer to the question of the step S504′ becomes affirmative(YES), which causes the photographing mode to be switched to the nightmode again. To prevent this, the first switching threshold value Yd1 isrequired to be higher than the luminance threshold value Yn. Therefore,the first switching threshold value Yd1 has a lower limit value setthereto.

If the difference Ydiff≧the difference threshold value Yth holds (i.e.if the difference is not smaller than the difference threshold value)(NO to the step S901), the camera control circuit 21 stores thedetermination result in the internal memory as a second determinationresult (step S905). Then, the camera control circuit 21 obtains theobject luminance Y1 detected according to the image signal from thephotometric circuit 26 again (step S906).

The step S906 is executed because the object luminance Y1 obtained inthe step S501 changes due to switching of the photographing mode to thenight mode in which light including near infrared light is received.

Then, the camera control circuit 21 calculates a second switchingthreshold value Yd2 with reference to which the photographing mode isswitched from the night mode to the day mode, based on the objectluminance Y1 obtained in the step S513 (step S907). Here, the cameracontrol circuit 21 calculates the second switching threshold value Yd2e.g. by an equation Yd2=Y1+α.

Similar to the first switching threshold value Yd1, the second switchingthreshold value Yd2 also has a lower limit value set thereto. After thestep S907, the camera control circuit 21 terminates the modedetermination process.

If infrared light is not being emitted by the infrared illuminationsection 11 (NO to the step S801), the camera control circuit 21 proceedsto the step S906. Further, if the object luminance Y1 the luminancethreshold value Yn holds (NO to the step S504′), the camera controlcircuit 21 terminates the mode determination process.

If the current photographing mode is not the day mode (NO to the stepS503), i.e. if the photographing mode is the night mode, the cameracontrol circuit 21 determines whether or not infrared light is beingemitted by the infrared illumination section 11 (step S804). If infraredlight is being emitted by the infrared illumination section 11 (YES tothe step S804), the camera control circuit 21 determines whether or notthe determination result stored in the internal memory is the firstdetermination result (step S908).

If the determination result stored in the internal memory is the firstdetermination result (YES to the step S908), the camera control circuit21 selects the object luminance Y2 as the selected luminance detectionresult, and compares the object luminance Y2 with the first switchingthreshold value Yd1 to thereby determine whether or not the objectluminance Y2>the first switching threshold value Yd1 holds (step S909).

If the object luminance Y2>the first switching threshold value Yd1 holds(YES to the step S909), the camera control circuit 21 switches thephotographing mode from the night mode to the day mode (step S910). Atthis time, in accordance with the switching from the night mode to theday mode, the camera control circuit 21 controls the infrared cut filtercontrol circuit 25 to insert the infrared cut filter 4 onto the opticalaxis 2. Then, the camera control circuit 21 terminates the modedetermination process.

If the object luminance Y2 the first switching threshold value Yd1 holds(NO to the step S909), the camera control circuit 21 terminates the modedetermination process.

If infrared light is not being emitted by the infrared illuminationsection 11 (NO to the step S804), the camera control circuit 21 selectsthe object luminance Y1 as the selected luminance detection result, andcompares the object luminance Y1 with the second switching thresholdvalue Yd2 to thereby determine whether or not the object luminanceY1>the second switching threshold value Yd2 holds (step S911). If theobject luminance Y1>the second switching threshold value Yd2 holds (YESto the step S911), the camera control circuit 21 proceeds to the stepS910 to switch the photographing mode from the night mode to the daymode.

On the other hand, if the object luminance Y1≦the second switchingthreshold value Yd2 holds (NO to the step S911), the camera controlcircuit 21 terminates the mode determination process.

If the determination result stored in the internal memory is not thefirst determination result (NO to the step S908), i.e. if thedetermination result stored in the internal memory is the seconddetermination result, the camera control circuit 21 proceeds to the stepS911 to determine whether or not the object luminance Y1>the secondswitching threshold value Yd2 holds.

As described above, in the first variation of the mode determinationprocess performed by the camera as the image pickup apparatus accordingto the second embodiment, in a case where infrared illumination is beingperformed, which object luminance is to be used in the switching controlfor switching the photographing mode from the night mode to the day modeis determined based on the difference Ydiff between the object luminanceY1 and the object luminance Y2.

In the first variation, if the difference Ydiff is smaller than thepredetermined difference threshold value Yth, it is judged that thedifference between the photographing range and the range of luminancedetection is small, and hence the reliability of the object luminance Y2is high, so that the camera control circuit 21 controls the switching ofthe photographing mode from the night mode to the day mode, based on theobject luminance Y2. That is, the camera control circuit 21 inserts theinfrared cut filter 4 onto the optical axis 2 based on the objectluminance Y2.

On the other hand, if the difference Ydiff is not smaller than thepredetermined difference threshold value Yth, it is judged that thedifference between the photographing range and the range of luminancedetection is large, and hence the reliability of the object luminance Y2is low, so that the camera control circuit 21 controls the switching ofthe photographing mode from the night mode to the day mode, based on theobject luminance Y1. That is, the camera control circuit 21 inserts theinfrared cut filter 4 onto the optical axis 2 based on the objectluminance Y1.

Further, in a case where infrared illumination is not being performed,an infrared component in a photographed image is less, and hence it isjudged that the reliability of the object luminance Y1 is high, and thecamera control circuit 21 controls the switching of the photographingmode from the night mode to the day mode, based on the object luminanceY1. That is, the camera control circuit 21 inserts the infrared cutfilter 4 onto the optical axis 2 based on the object luminance Y1.

This makes it possible to perform the mode switching control using theobject luminance Y1 or Y2 suitable for the photographic scene. That is,it is possible to accurately switch the photographing mode even when theinfrared light reflectivity is different. Further, it is possible toaccurately and stably insert and remove the infrared cut filter 4.

Although in the above-described first variation, the difference iscalculated when comparing the object luminance Y1 and the objectluminance Y2 with each other, in place of the difference, a ratiobetween the object luminance Y1 and the object luminance Y2, forexample, may be used.

More specifically, in a case where a first determination result isobtained which indicates that the ratio between the object luminance Y1and the object luminance Y2 is lower than a ratio threshold value set inadvance, the camera control circuit 21 performs the control ofinsertion/removal of the IR cut filter based on the object luminance Y2.Further, in a case where a second determination result is obtained whichindicates that the above-mentioned ratio is not lower than the ratiothreshold valued, the camera control circuit 21 performs the control ofinsertion/removal of the IR cut filter based on the object luminance Y1.

FIG. 10 is a flowchart of a second variation of the mode determinationprocess for the mode switching operation performed by the camera as theimage pickup apparatus according to the second embodiment.

The same steps of the mode determination process in FIG. 10 as those ofthe mode determination process in FIG. 9 are denoted by the same stepnumbers, and description thereof is omitted.

If it is determined in the step S801 that infrared illumination isperformed (YES to the step S801), the camera control circuit 21determines whether or not the difference Ydiff<the difference thresholdvalue Yth or the object luminance Y1>the object luminance Y2 holds (stepS1001). If the difference Ydiff<the difference threshold value Yth orthe object luminance Y1>the object luminance Y2 holds (YES to the stepS1001), the camera control circuit 21 proceeds to the step S902, andstores the determination result in the internal memory as the firstdetermination result.

On the other hand, if the difference Ydiff the difference thresholdvalue Yth holds and also the object luminance Y1 the object luminance Y2holds (the first detection result is not higher than the seconddetection result) (NO to the step S1001), the camera control circuit 21proceeds to the step S905, and stores the determination result in theinternal memory as the second determination result.

As described above, in the second variation, even when the differenceYdiff between the object luminance Y1 and the object luminance Y2 is notsmaller than the difference threshold value Yth, if the object luminanceY1 is higher than the object luminance Y2, it is judged that the objectluminance Y2 which is still lower than the object luminance Y1 which islower than the luminance threshold value Yn is substantially equivalentto the object luminance Y1, and the reliability of the object luminanceY2 is high. Then, the camera control circuit 21 controls the switchingof the photographing mode from the night mode to the day mode based onthe object luminance Y2. That is, the camera control circuit 21 insertsthe infrared cut filter 4 onto the optical axis 2 based on the objectluminance Y2.

With this, in the second embodiment, it is possible to perform the modeswitching control using the object luminance suitable for thephotographic scene with less influence by illumination with infraredlight than in the first embodiment. That is, it is possible toaccurately switch the photographing mode even when the infrared lightreflectivity is different.

Note that although in the above-described second variation as well, thedifference is calculated when comparing the object luminance Y1 and theobject luminance Y2 with each other, in place of the difference, a ratiobetween the object luminance Y1 and the object luminance Y2, forexample, may be used.

More specifically, in a case where a first determination result isobtained which indicates either that the ratio between the objectluminance Y1 and the object luminance Y2 is lower than a ratio thresholdvalue set in advance, or that the object luminance Y1 is higher than theobject luminance Y2, the camera control circuit 21 performs the controlof insertion/removal of the IR cut filter based on the object luminanceY2. On the other hand, in a case where a second determination result isobtained which indicates that the above-mentioned ratio is not lowerthan the ratio threshold value, and also that the object luminance Y1 isnot higher than the object luminance Y2, the camera control circuit 21performs the control of insertion/removal of the IR cut filter based onthe object luminance Y1.

As is clear from the above description, in the second embodiment, thephotometric circuit 26 functions as the first detection unit, and theluminance sensor 8 and the luminance sensor control circuit 23 functionas the second detection unit. Further, the camera control circuit 21functions as a selection unit and the mode switching control unit, andthe camera control circuit 21 and the infrared cut filterinsertion/removal motor 6 function as the insertion/removal controlunit.

Next, a description will be given of a third embodiment of the presentinvention. FIG. 11 is a schematic diagram of a camera as an image pickupapparatus according to the third embodiment. FIG. 12 is a block diagramof the control circuit 10 appearing in FIG. 11. The third embodimentdiffers from the second embodiment in that the luminance sensor 8appearing in FIG. 6 is omitted in FIG. 11, and the luminance sensor 8and the luminance sensor control circuit 23 appearing in FIG. 7 areomitted in FIG. 12.

FIG. 13 is a flowchart of a mode determination process for the modeswitching operation performed by the camera shown in FIG. 11. The modedetermination process in FIG. 13 is performed under the control of thecamera control circuit 21, and is, for example, automatically andrepeatedly executed when the camera is in the power-on state.

When the mode determination process is started, the camera controlcircuit 21 obtains a luminance detection result (object luminance Y1)from the photometric circuit 26 (step S1301). Then, the camera controlcircuit 21 determines whether or not the current photographing mode isthe day mode (i.e. a color photographing mode) (step S1302).

If the current photographing mode is the day mode (YES to the stepS1302), the camera control circuit 21 compares the object luminance Y1with a first switching threshold value TH_(DN) set in advance, anddetermines whether or not the object luminance Y1<the first switchingthreshold value TH_(DN) holds (step S1303).

The first switching threshold value TH_(DN) is a threshold value withreference to which the photographing mode is switched from the day modeto the night mode.

If the object luminance Y1<the first switching threshold value TH_(DN)holds (i.e. the object luminance Y1 is lower than the first switchingthreshold value) (YES to the step S1303), the camera control circuit 21switches the photographing mode from the day mode to the night mode(i.e. the monochrome photographing mode) (step S1304). In accordancewith the switching from the day mode to the night mode, the cameracontrol circuit 21 controls the infrared cut filter control circuit 25to remove the infrared cut filter 4 from the optical axis 2.

Then, the camera control circuit 21 obtains a luminance detection resultfrom the photometric circuit 26 as the object luminance Y2 (step S1305).As mentioned above, since the infrared cut filter is removed and thesensitivity is increased in the night mode, the object luminance in thenight mode is set as Y2. Note that under a photographic environmentcontaining infrared light, the object luminance Y2 is sometimes notlower than the object luminance Y1.

Next, the camera control circuit 21 causes the infrared illuminationcontrol circuit 28 to light the infrared illumination section 11 so asto illuminate the object with infrared light (step S1306). Then, thecamera control circuit 21 obtains the intensity of light emission by theinfrared illumination section 11 from the infrared illumination controlcircuit 28 as a light emission intensity P1 (step S1307).

Then, the camera control circuit 21 obtains a luminance detection resultfrom the photometric circuit 26 as an object luminance Y3 (step S1308).Here, since the object is illuminated with infrared light, the objectluminance under infrared illumination is set as the object luminance Y3.Note that the object luminance Y3 under infrared illumination isgenerally not lower than the object luminance Y2.

After that, the camera control circuit 21 calculates a differencebetween the object luminance Y2 and the object luminance Y3, and setsthe difference as an object luminance rise value Yup which is anincrease in object luminance caused by illumination with infrared light(step S1309).

Then, the camera control circuit 21 calculates a second switchingthreshold value TH_(ND1) with reference to which the photographing modeis switched from the night mode to the day mode, based on the objectluminance Y2 (step S1310). Then, the camera control circuit 21terminates the mode determination process.

The second switching threshold value TH_(ND1) is calculated e.g. by anequation TH_(ND1)=Y2+α. The variable α represents hysteresis forpreventing occurrence of hunting in switching the photographing mode,and when the variable α is set to a larger value, it is possible to moreproactively prevent occurrence of hunting. On the other hand, when thevariable α is set to a smaller value, it is possible to more quicklyswitch the photographing mode from the night mode to the day mode.

If the object luminance Y1 the first switching threshold value TH_(DN)holds (the object luminance Y1 is not lower than the first switchingthreshold value) (NO to the step S1303), the camera control circuit 21terminates the mode determination process.

If the current photographing mode is not the day mode (NO to the stepS1302), i.e. if the current photographing mode is the night mode, thecamera control circuit 21 obtains the intensity of light emission by theinfrared illumination section 11 from the infrared illumination controlcircuit 28 as a light emission intensity P2 (step S1311).

If there is a change in intensity of light emission by the infraredillumination section 11 after switching the photographing mode to thenight mode, the light emission intensity P2 is different from the lightemission intensity P1. On the other hand, if there is no change inintensity of light emission by the infrared illumination section 11after switching the photographing mode to the night mode, the lightemission intensity P2 is equal to the light emission intensity P1.

Then, the camera control circuit 21 corrects the second switchingthreshold value TH_(ND1) obtained in the step S1310, and sets thecorrected threshold value as a third switching threshold value TH_(ND2)(step S1312). Here, the camera control circuit 21 corrects the secondswitching threshold value TH_(ND1) by adding an increase value(correction value) of the object luminance caused by illumination withinfrared light. In doing this, the correction value becomes larger asthe intensity of light emission by the infrared illumination section 11is higher, and becomes smaller as the intensity of light emission islower.

When the light emission intensity P2 is equal to 0 (unlighted state),the correction value becomes equal to 0, and when the light emissionintensity P2 is equal to the light emission intensity P1, the correctionvalue becomes equal to the object luminance rise value Yup correspondingto an amount of increase in the object luminance. Therefore, the thirdswitching threshold value TH_(ND2) is calculated e.g. by an equationTH_(ND2)=TH_(ND1)+(Yup×P2/P1). Note that P2/P1 represents a rate ofchange of the light emission intensity.

The camera control circuit 21 may perform correction based on acorrection data table recorded in the internal memory or the like inadvance. In this correction data table, for example, a correction valuefor correction is defined according to the relationship between theobject luminance and the light emission intensity.

Further, the amount of increase in the object luminance caused byillumination with infrared light also varies with a distance to anobject, temperature of the infrared illumination section 11, and agedeterioration (i.e. aging) of the infrared illumination section 11, andhence it is possible to more accurately perform correction by takinginto account these factors.

Then, the camera control circuit 21 compares the object luminance Y1with the third switching threshold value TH_(ND2), and determineswhether or not the object luminance Y1>the third switching thresholdvalue TH_(ND2) holds (step S1313). If the object luminance Y1>the thirdswitching threshold value TH_(ND2) holds (YES to the step S1313), thecamera control circuit 21 causes the infrared illumination controlcircuit 28 to extinguish the infrared illumination section 11 (stepS1314). That is, the camera control circuit 21 causes light emission bythe infrared illumination section 11 to be stopped.

After that, the camera control circuit 21 switches the photographingmode from the night mode to the day mode (step S1315). In doing this, inaccordance with the switching from the night mode to the day mode, thecamera control circuit 21 controls the infrared cut filter controlcircuit 25 to insert the infrared cut filter 4 onto the optical axis 2.Then, the camera control circuit 21 terminates the mode determinationprocess.

If the object luminance Y1 the third switching threshold value TH_(ND2)holds (NO to the step S1313), the camera control circuit 21 terminatesthe mode determination process.

As described above, in the third embodiment, the second switchingthreshold value with reference to which the photographing mode isswitched from the night mode to the day mode is corrected according tothe intensity of light emission by the infrared illumination section 11,and hence it is possible to prevent the photographing mode from beingunexpectedly switched due to the influence of the light emissionintensity.

Although in the above description, in the step S1312, the secondswitching threshold value with reference to which the photographing modeis switched from the night mode to the day mode is corrected by addingan amount of increase in the object luminance caused by illuminationwith infrared light, the second switching threshold value may becorrected by subtracting the amount of increase in the object luminancecaused by illumination with infrared light from the object luminance.

As is clear from the above, in the third embodiment, the photometriccircuit 26 functions as a detection unit, and the camera control circuit21 functions as a correction unit. Further, the camera control circuit21 and the infrared cut filter insertion/removal motor 6 function as acontrol unit.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

OTHER EMBODIMENTS

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

This application claims the benefit of Japanese Patent Application No.2014-220205 filed Oct. 29, 2014, No. 2014-220206 filed Oct. 29, 2014,and No. 2014-220204 filed Oct. 29, 2014, which are hereby incorporatedby reference herein in their entirety.

What is claimed is:
 1. An image pickup apparatus including an imagepickup device that has an optical image formed thereon through aphotographic optical system, and outputs an image signal correspondingto the optical image, comprising: an infrared light reduction unitconfigured to be inserted between the photographic optical system andthe image pickup device so as to reduce an amount of infrared lightcontained in the optical image reaching the image pickup device; a firstdetection unit configured to detect a luminance of an object accordingto the image signal to thereby obtain a first detection result; a seconddetection unit disposed at a location different from a location of theimage pickup device and configured to detect a luminance of the objectto thereby obtain a second detection result; a processing unitconfigured to, in a case where the first detection result is lower thana first switching threshold value set in advance, generate a secondswitching threshold value used for comparison with the first detectionresult and a third switching threshold value used for comparison withthe second detection result, based on the first detection result, thesecond detection result, and the first switching threshold value, andmake one of the second switching threshold value and the third switchingthreshold value equal to or higher than the other of the secondswitching threshold value and the third switching threshold value; andan insertion/removal control unit configured to performinsertion/removal control for selectively inserting or removing saidinfrared light reduction unit into or from an optical path between thephotographic optical system and the image pickup device, based on thefirst detection result, the second detection result, the secondswitching threshold value, and the third switching threshold value. 2.The image pickup apparatus according to claim 1, wherein in a case wherea first determination result is obtained which indicates either that adifference between the first detection result and the second detectionresult is smaller than a difference threshold value set in advance, orthat the first detection result is higher than the second detectionresult, said processing unit performs first processing for generatingthe second switching threshold value based on the first switchingthreshold value, generating the third switching threshold value based onthe second detection result, and making the third switching thresholdvalue equal to or higher than the second switching threshold value. 3.The image pickup apparatus according to claim 2, wherein in a case wherea second determination result is obtained which indicates that thedifference between the first detection result and the second detectionresult is not smaller than the difference threshold value set inadvance, and also that the first detection result is not higher than thesecond detection result, said processing unit performs second processingfor generating the third switching threshold value based on the firstswitching threshold value, generating the second switching thresholdvalue based on the first detection result, and making the secondswitching threshold value equal to or higher than the third switchingthreshold value.
 4. The image pickup apparatus according to claim 1,wherein in a case where a first determination result is obtained whichindicates either that a ratio between the first detection result and thesecond detection result is lower than a ratio threshold value set inadvance, or that the first detection result is higher than the seconddetection result, said processing unit performs first processing forgenerating the second switching threshold value based on the firstswitching threshold value, generating the third switching thresholdvalue based on the second detection result, and making the thirdswitching threshold value equal to or higher than the second switchingthreshold value.
 5. The image pickup apparatus according to claim 4,wherein in a case where a second determination result is obtained whichindicates that the ratio between the first detection result and thesecond detection result is not lower than the ratio threshold value setin advance, and also that the first detection result is not higher thanthe second detection result, said processing unit performs secondprocessing for generating the third switching threshold value based onthe first switching threshold value, generating the second switchingthreshold value based on the first detection result, and making thesecond switching threshold value equal to or higher than the thirdswitching threshold value.
 6. The image pickup apparatus according toclaim 1, comprising a mode switching control unit configured to switch aphotographing mode to a color photographing mode in accordance withinsertion of said infrared light reduction unit into the optical path,and switch the photographing mode to a monochrome photographing mode inaccordance with removal of said infrared light reduction unit from theoptical path.
 7. The image pickup apparatus according to claim 6,wherein in a case where the first detection result is higher than thesecond switching threshold value, and also the second detection resultis higher than the third switching threshold value, said mode switchingcontrol unit switches the photographing mode from the colorphotographing mode to the monochrome photographing mode.
 8. The imagepickup apparatus according to claim 6, wherein in a case where thephotographing mode is the color photographing mode, when the firstdetection result is lower than the first switching threshold value, saidmode switching control unit switches the photographing mode to themonochrome photographing mode.
 9. The image pickup apparatus accordingto claim 3, comprising an illuminating unit configured to illuminate theobject with infrared light, wherein in a case where the photographingmode is the monochrome photographing mode in which said infrared lightreduction unit is removed from the optical path, when illumination bysaid illuminating unit is being performed, said processing unit performsthe first or second processing in accordance with the firstdetermination result or the second determination result.
 10. The imagepickup apparatus according to claim 9, wherein in a case where thephotographing mode is the monochrome photographing mode, whenillumination by said illuminating unit is not being performed, saidprocessing unit performs third processing for generating the secondswitching threshold value based on the first detection result.
 11. Theimage pickup apparatus according to claim 9, comprising a mode switchingcontrol unit configured to, in a case where the photographing mode isthe monochrome photographing mode, when illumination by saidilluminating unit is being performed, change the photographing mode fromthe monochrome mode to the color photographing mode in which saidinfrared light reduction unit is inserted into the optical path in acase where the first detection result is higher than the secondswitching threshold value, and also the second detection result ishigher than the third switching threshold value.
 12. The image pickupapparatus according to claim 9, comprising a mode switching control unitconfigured to, in a case where the photographing mode is the monochromephotographing mode, when illumination by said illuminating unit is notbeing performed, change the photographing mode from the monochrome modeto the color photographing mode in which said infrared light reductionunit is inserted into the optical path in a case where the firstdetection result is higher than the second switching threshold value.13. An image pickup apparatus including an image pickup device that hasan optical image formed thereon through a photographic optical system,and outputs an image signal corresponding to the optical image,comprising: an infrared light reduction unit configured to be insertedbetween the photographic optical system and the image pickup device soas to reduce an amount of infrared light contained in the optical imagereaching the image pickup device; a first detection unit configured todetect a luminance of an object according to the image signal to therebyobtain a first detection result; a second detection unit disposed at alocation different from a location of the image pickup device andconfigured to detect a luminance of the object to thereby obtain asecond detection result; a selection unit configured to compare thefirst detection result and the second detection result, and select oneof the first detection result and the second detection result as aselected detection result based on a comparison result; and aninsertion/removal control unit configured to perform insertion/removalcontrol for selectively inserting or removing said infrared lightreduction unit into or from an optical path between the photographicoptical system and the image pickup device according to the selecteddetection result.
 14. The image pickup apparatus according to claim 13,wherein in a case where a first determination result is obtained whichindicates that a difference between the first detection result and thesecond detection result is smaller than a difference threshold value setin advance, said selection unit selects the second detection result asthe selected detection result, whereas in a case where a seconddetermination result is obtained which indicates that the difference isnot smaller than the difference threshold value, said selection unitselects the first detection result as the selected detection result. 15.The image pickup apparatus according to claim 13, wherein in a casewhere a first determination result is obtained which indicates eitherthat a difference between the first detection result and the seconddetection result is smaller than a difference threshold value set inadvance, or that the first detection result is higher than the seconddetection result, said selection unit selects the second detectionresult as the selected detection result, whereas in a case where asecond determination result is obtained which indicates that thedifference is not smaller than the difference threshold value, and alsothat the first detection result is not higher than the second detectionresult is obtained, said selection unit selects the first detectionresult as the selected detection result.
 16. The image pickup apparatusaccording to claim 13, wherein in a case where a first determinationresult is obtained which indicates that a ratio between the firstdetection result and the second detection result is lower than a ratiothreshold value set in advance, said selection unit selects the seconddetection result as the selected detection result, whereas in a casewhere a second determination result is obtained which indicates that theratio is not lower than the ratio threshold value, said selection unitselects the first detection result as the selected detection result. 17.The image pickup apparatus according to claim 13, wherein in a casewhere a first determination result is obtained which indicates eitherthat a ratio between the first detection result and the second detectionresult is lower than a ratio threshold value set in advance, or that thefirst detection result is higher than the second detection result isobtained, said selection unit selects the second detection result as theselected detection result, whereas in a case where a seconddetermination result is obtained which indicates that the ratio is notlower than the ratio threshold value, and also that the first detectionresult is not higher than the second detection result is obtained, saidselection unit selects the first detection result as the selecteddetection result.
 18. The image pickup apparatus according to claim 13,comprising a mode switching control unit configured to switch aphotographing mode to a color photographing mode in accordance withinsertion of said infrared light reduction unit into the optical path,and switch the photographing mode to a monochrome photographing mode inaccordance with removal of said infrared light reduction unit from theoptical path.
 19. The image pickup apparatus according to claim 18,wherein in a case where the photographing mode is the colorphotographing mode, when the first detection result is lower than apredetermined luminance threshold value, said mode switching controlunit switches the photographing mode to the monochrome photographingmode.
 20. The image pickup apparatus according to claim 18, comprisingan illuminating unit configured to illuminate the object with infraredlight, wherein in a case where the photographing mode is the monochromephotographing mode, said selection unit selects the second detectionresult as the selected detection result in a case where the firstdetermination result is obtained when illumination by said illuminatingunit is being performed, and selects the first detection result as theselected detection result in a case where the second determinationresult is obtained when illumination by said illuminating unit is beingperformed.
 21. The image pickup apparatus according to claim 20, whereinin a case where the photographing mode is the monochrome photographingmode, in a case where illumination by said illuminating unit is notbeing performed, said selection unit selects the first detection resultas the selected detection result.
 22. An image pickup apparatusincluding an image pickup device that has an optical image formedthereon through a photographic optical system, and outputs an imagesignal corresponding to the optical image, comprising: an infrared lightreduction unit configured to be inserted into an optical path betweenthe photographic optical system and the image pickup device so as toreduce an amount of infrared light contained in the optical imagereaching the image pickup device; a detection unit configured to detecta luminance of an object according to the image signal to thereby obtaina luminance detection result; an illuminating unit configured toilluminate the object with infrared light; a correction unit configuredto correct a threshold value for use in determining insertion or removalof said infrared light reduction unit into or from the optical path,based on a light emission intensity of infrared light emitted by saidilluminating unit; and a control unit configured to compare theluminance detection result and a corrected threshold value, anddetermine insertion of said infrared light reduction unit into theoptical path according to the comparison result.
 23. The image pickupapparatus according to claim 22, wherein said correction unit correctsthe threshold value such that the threshold value is made higher as thelight emission intensity is higher.
 24. The image pickup apparatusaccording to claim 22, wherein said correction unit corrects thedetection result obtained from said detection unit in a state where saidinfrared light reduction unit is removed from the optical path betweenthe photographic optical system and the image pickup device instead ofcorrecting the threshold value, and wherein said control unit comparesthe corrected detection result and the threshold value, and determinesinsertion of said infrared light reduction unit into the optical pathaccording to the comparison result.
 25. The image pickup apparatusaccording to claim 24, wherein said correction unit corrects thedetection result such that the detection result is made lower as thelight emission intensity is higher.
 26. The image pickup apparatusaccording to claim 22, wherein said correction unit calculates acorrection value for correction, based on a rate of change of the lightemission intensity and an amount of increase in the detection result.27. The image pickup apparatus according to claim 22, wherein saidcorrection unit calculates a correction value for correction, using acorrection data table in which the correction value for correction isdefined according to a relationship between the detection result and thelight emission intensity.
 28. The image pickup apparatus according toclaim 22, wherein said correction unit performs the correction by takinginto account at least one of a distance from the image pickup apparatusto an object, temperature of said illuminating unit, and aging of saidilluminating unit.
 29. The image pickup apparatus according to claim 22,wherein said control unit switches the photographing mode to the colorphotographing mode in a case where said infrared light reduction unit isinserted into the optical path, and switches the photographing mode tothe monochrome photographing mode in a case where said infrared lightreduction unit is removed from the optical path.
 30. The image pickupapparatus according to claim 29, wherein in a case where the detectionresult is lower than the threshold value, said control unit changes thephotographing mode from the color photographing mode to the monochromephotographing mode.
 31. The image pickup apparatus according to claim29, wherein said control unit caused said illuminating unit to performillumination when in the monochrome photographing mode.
 32. The imagepickup apparatus according to claim 29, wherein in a case where thedetection result is higher than the corrected threshold value, saidcontrol unit causes said illuminating unit to stop illumination, andswitches the photographing mode from the monochrome photographing modeto the color photographing mode.
 33. A method of controlling an imagepickup apparatus including an image pickup device that has an opticalimage formed thereon through a photographic optical system, and outputsan image signal corresponding to the optical image, and an infraredlight reduction unit configured to be inserted between the photographicoptical system and the image pickup device so as to reduce an amount ofinfrared light contained in the optical image reaching the image pickupdevice, the method comprising: detecting a luminance of an objectaccording to the image signal to thereby obtain a first detectionresult; detecting a luminance of the object using a luminance sensordisposed at a location different from a location of the image pickupdevice to thereby obtain a second detection result; generating, in acase where the first detection result is lower than a first switchingthreshold value set in advance, a second switching threshold value usedfor comparison with the first detection result and a third switchingthreshold value used for comparison with the second detection result,based on the first detection result, the second detection result, andthe first switching threshold value, and making one of the secondswitching threshold value and the third switching threshold value equalto or higher than the other of the second switching threshold value andthe third switching threshold value; and performing insertion/removalcontrol for selectively inserting or removing the infrared lightreduction unit into or from an optical path between the photographicoptical system and the image pickup device, based on the first detectionresult, the second detection result, the second switching thresholdvalue, and the third switching threshold value.
 34. A method ofcontrolling an image pickup apparatus including an image pickup devicethat has an optical image formed thereon through a photographic opticalsystem, and outputs an image signal corresponding to the optical image,and an infrared light reduction unit configured to be inserted betweenthe photographic optical system and the image pickup device so as toreduce an amount of infrared light contained in the optical imagereaching the image pickup device, the method comprising: detecting aluminance of an object according to the image signal to thereby obtain afirst detection result; detecting a luminance of the object using aluminance sensor disposed at a location different from a location of theimage pickup device to thereby obtain a second detection result;comparing the first detection result and the second detection result,and selecting one of the first detection result and the second detectionresult as a selected detection result based on a comparison result; andperforming insertion/removal control for selectively inserting orremoving the infrared light reduction unit into or from an optical pathbetween the photographic optical system and the image pickup deviceaccording to the selected detection result.
 35. A method of controllingan image pickup apparatus including an image pickup device that has anoptical image formed thereon through a photographic optical system, andoutputs an image signal corresponding to the optical image, an infraredlight reduction unit configured to be inserted into an optical pathbetween the photographic optical system and the image pickup device soas to reduce an amount of infrared light contained in the optical imagereaching the image pickup device, and an illuminating unit configured toilluminate the object with infrared light, the method comprising:detecting a luminance of an object according to the image signal tothereby obtain a luminance detection result; correcting a thresholdvalue for use in determining insertion or removal of the infrared lightreduction unit into or from the optical path, based on a light emissionintensity of infrared light emitted by the illuminating unit; andcomparing the detection result and a corrected threshold value, anddetermining insertion of the infrared light reduction unit into theoptical path according to the comparison result.
 36. A non-transitorycomputer-readable storage medium storing a computer-executable controlprogram for causing a computer to execute a method of controlling animage pickup apparatus including an image pickup device that has anoptical image formed thereon through a photographic optical system, andoutputs an image signal corresponding to the optical image, and aninfrared light reduction unit configured to be inserted between thephotographic optical system and the image pickup device so as to reducean amount of infrared light contained in the optical image reaching theimage pickup device, wherein the method comprises: detecting a luminanceof an object according to the image signal to thereby obtain a firstdetection result; detecting a luminance of the object using a luminancesensor disposed at a location different from a location of the imagepickup device to thereby obtain a second detection result; generating,in a case where the first detection result is lower than a firstswitching threshold value set in advance, a second switching thresholdvalue used for comparison with the first detection result and a thirdswitching threshold value used for comparison with the second detectionresult, based on the first detection result, the second detectionresult, and the first switching threshold value, and making one of thesecond switching threshold value and the third switching threshold valueequal to or higher than the other of the second switching thresholdvalue and the third switching threshold value; and performinginsertion/removal control for selectively inserting or removing theinfrared light reduction unit into or from an optical path between thephotographic optical system and the image pickup device, based on thefirst detection result, the second detection result, the secondswitching threshold value, and the third switching threshold value. 37.A non-transitory computer-readable storage medium storing acomputer-executable control program for causing a computer to execute amethod of controlling an image pickup apparatus including an imagepickup device that has an optical image formed thereon through aphotographic optical system, and outputs an image signal correspondingto the optical image, and an infrared light reduction unit configured tobe inserted between the photographic optical system and the image pickupdevice so as to reduce an amount of infrared light contained in theoptical image reaching the image pickup device, wherein the methodcomprises: detecting a luminance of an object according to the imagesignal to thereby obtain a first detection result; detecting a luminanceof the object using a luminance sensor disposed at a location differentfrom a location of the image pickup device to thereby obtain a seconddetection result; comparing the first detection result and the seconddetection result, and selecting one of the first detection result andthe second detection result as a selected detection result based on acomparison result; and performing insertion/removal control forselectively inserting or removing the infrared light reduction unit intoor from an optical path between the photographic optical system and theimage pickup device according to the selected detection result.
 38. Anon-transitory computer-readable storage medium storing acomputer-executable control program for causing a computer to execute amethod of controlling an image pickup apparatus including an imagepickup device that has an optical image formed thereon through aphotographic optical system, and outputs an image signal correspondingto the optical image, an infrared light reduction unit configured to beinserted into an optical path between the photographic optical systemand the image pickup device so as to reduce an amount of infrared lightcontained in the optical image reaching the image pickup device, and anilluminating unit configured to illuminate the object with infraredlight, wherein the method comprises: detecting a luminance of an objectaccording to the image signal to thereby obtain a luminance detectionresult; correcting a threshold value for use in determining insertion orremoval of the infrared light reduction unit into or from the opticalpath, based on a light emission intensity of infrared light emitted bythe illuminating unit; and comparing the detection result and acorrected threshold value, and determining insertion of the infraredlight reduction unit into the optical path according to the comparisonresult.